Wolbachia is a widespread group of intracellular bacteria commonly found in arthropods. In many insect species, Wolbachia induce a cytoplasmic mating incompatibility (CI). If different Wolbachia infections occur in the same host species, bidirectional CI is often induced. Bidirectional CI acts as a postzygotic isolation mechanism if parapatric host populations are infected with different Wolbachia strains. Therefore, it has been suggested that Wolbachia could promote speciation in their hosts. In this article we investigate theoretically whether Wolbachia-induced bidirectional CI selects for premating isolation and therefore reinforces genetic divergence between parapatric host populations. To achieve this we combined models for Wolbachia dynamics with a well-studied reinforcement model. This new model allows us to compare the effect of bidirectional CI on the evolution of female mating preferences with a situation in which postzygotic isolation is caused by nuclear genetic incompatibilities (NI). We distinguish between nuclear incompatibilities caused by two loci with epistatic interactions, and a single locus with incompatibility among heterozygotes in the diploid phase. Our main findings are: (1) bidirectional CI and single locus NI select for premating isolation with a higher speed and for a wider parameter range than epistatic NI; (2) under certain parameter values, runaway sexual selection leads to the increase of an introduced female preference allele and fixation of its preferred male trait allele in both populations, whereas under others it leads to divergence in the two populations in preference and trait alleles; and (3) bidirectional CI and single locus NI can stably persist up to migration rates that are two times higher than seen for epistatic NI. The latter finding is important because the speed with which mutants at the preference locus spread increases exponentially with the migration rate. In summary, our results show that bidirectional CI selects for rapid premating isolation and so generally support the view that Wolbachia can promote speciation in their hosts.

When two mutations are singly deleterious but neutral or beneficial together, compensatory evolution can occur. The accumulation of derived, compensated genotypes contributes to the evolution of genetic incompatibilities between diverging populations or species. Previous two locus/two allele models have shown that compensatory evolution is appreciable only with tight linkage, the possibility of nearly simultaneous mutations, and/or a way to overcome negative selection against the singly mutated genotype. These conditions are often not met. Even when they are met, compensatory evolution is still predicted to be extremely slow, and in many scenarios selective advantage of the compensated genotype does little to accelerate it. Despite these obstacles, empirical studies suggest that it occurs readily. We describe here a set of related two locus/three allele models that invoke plausible neutral intermediates capable of productive interaction with both ancestral and compensated products of the interacting locus. These models are explored with analytical and computer simulation methods. The effect of these stepping-stone alleles on the evolution of ancestor-descendant incompatibilities is often profound, making the difference between evolution and stasis in several situations, including in small populations, when codominance or haploidy prevents shielding of mismatched genotypes, and in the absence of positive selection on the derived genotype. However, in large populations these intermediates can either speed or slow the evolution of incompatible genotypes relative to the two-allele case, depending on the specific fitness model. These results suggest that population size, the source of adaptive benefit, and the structural details of heteromeric gene product complexes interact to influence the path by which intergenic incompatibility evolves.

Interpretation of genetic differentiation values is often problematic because of their dependence on the level of genetic variation. For example, the maximum level of G_ST is less than the average within population homozygosity so that for highly variable loci, even when no alleles are shared between subpopulations, G_ST may be low. To remedy this difficulty, a standardized measure of genetic differentiation is introduced here, one which has the same range, 0–1, for all levels of genetic variation. With this measure, the magnitude is the proportion of the maximum differentiation possible for the level of subpopulation homozygosity observed. This is particularly important for situations in which the mutation rate is of the same magnitude or higher than the rate of gene flow. The standardized measure allows comparison between loci with different levels of genetic variation, such as allozymes and microsatellite loci, or mtDNA and Y-chromosome genes, and for genetic differentiation for organisms with different effective population sizes.

We examine the evolution of mesic forest ecosystems in the Pacific Northwest of North America using a statistical phylogeography approach in four animal and two plant lineages. Three a priori hypotheses, which explain the disjunction in the mesic forest ecosystem with either recent dispersal or ancient vicariance, are tested with phylogenetic and coalescent methods. We find strong support in three amphibian lineages (Ascaphus spp., and Dicampton spp., and Plethodon vandykei and P. idahoensis) for deep divergence between coastal and inland populations, as predicted by the ancient vicariance hypothesis. Unlike the amphibians, the disjunction in other Pacific Northwest lineages is likely due to recent dispersal along a northern route. Topological and population divergence tests support the northern dispersal hypothesis in the water vole (Microtus richardsoni) and northern dispersal has some support in both the dusky willow (Salix melanopsis) and whitebark pine (Pinus albicaulis). These analyses demonstrate that genetic data sampled from across an ecosystem can provide insight into the evolution of ecological communities and suggest that the advantages of a statistical phylogeographic approach are most pronounced in comparisons across multiple taxa in a particular ecosystem. Genetic patterns in organisms as diverse as willows and salamanders can be used to test general regional hypotheses, providing a consistent metric for comparison among members of an ecosystem with disparate life-history traits.

An understanding of the evolution of modern terrestrial ecosystems requires an understanding of the dynamics associated with angiosperm evolution, including the timing of their origin and diversification into their extraordinary present-day diversity. Molecular estimates of angiosperm age have varied widely, and many substantially predate the Early Cretaceous fossil appearance of the group. In this study, the effect of different genes, codon positions, and chronological constraints on node ages are examined on divergence time estimates across seed plants, with a special focus on angiosperms. Penalized likelihood was used to estimate divergence times on a phylogenetic hypothesis for seed plants derived from Bayesian analysis, with branch lengths estimated with maximum likelihood. The plastid genes atpB, psaA, psbB, and rbcL were used individually and in combination, using first and second, third, and the three codon positions, including and excluding age constraints on 20 nodes derived from a critical examination of the land-plant fossil record. The optimal level of rate smoothing according to each unconstrained and constrained dataset was obtained with penalized likelihood. Tests for a molecular clock revealed significantly unclocklike rates in all datasets. Addition of fossil constraints resulted in even greater departures from constancy. Consistently with significant deviations from a clock, estimated optimal smoothing values were low, but a strict correlation between rate heterogeneity and optimal smoothing value was not found. Age estimates for nodes across the phylogeny varied, sometimes substantially, with gene and codon position. Nevertheless, estimates based on the four concatenated genes are very similar to the mean of the four individual gene estimates. For any given node, unconstrained age estimates are more variable than constrained estimates and are frequently younger than well-substantiated fossil members of the clade. Constrained estimates of ages of clades are older than unconstrained estimates and oldest fossil representatives, sometimes substantially so. Angiosperm age estimates decreased as rate smoothing increased. Whereas the range of unconstrained angiosperm age estimates spans the fossil age of the clade, the range of constrained estimates is narrower (and older) than the earliest angiosperm fossils. Results unambiguously indicate the relevance of constraints in reducing the variability of ages derived from different partitions of the data and diminishing the effect of the smoothing parameter. Constrained optimizations of divergence times and substitution rates across the phylogeny suggest appreciably different evolutionary dynamics for angiosperms and for gymnosperms. Whereas the gymnosperm crown group originated shortly after the origin of seed plants, a long time elapsed before the origin of crown group angiosperms. Although absolute age estimates of angiosperms and angiosperm clades are older than their earliest fossils, the estimated pace of phylogenetic diversification largely agrees with the rapid appearance of angiosperm lineages in stratigraphic sequences.

Every species occupies a limited geographic area, but it remains unclear why traits that limit distribution do not evolve to allow range expansion. Hypotheses for the evolutionary stability of geographic ranges assume that species are maladapted at the range boundary and unfit beyond the current range, but this assumption has rarely been tested. To examine how fitness varies across species' ranges, we reciprocally transplanted two species of monkeyflowers, Mimulus cardinalis and M. lewisii, within and beyond their present elevation ranges. We used individuals of known parentage from populations collected across the elevation ranges of both species to examine whether populations are adapted to position within the range. For both species we found the greatest average fitness at elevations central within the range, reduced fitness at the range margin, and zero or near-zero fitness when transplanted beyond their present elevation range limits. However, the underlying causes of fitness variation differed between the species. At high elevations beyond its range, M. cardinalis displayed reduced growth and fecundity, whereas at low elevations M. lewisii experienced high mortality. Weak differences in performance were observed among populations within each species and these were not related to elevation of origin. Low fitness of both species at their range margin and weak differentiation among populations within each species suggest that adaptation to the environment at and beyond the range margin is hindered, illustrating that range margins provide an interesting system in which to study limits to adaptation.

Data from four DNA regions (rbcL, matK, 26S rDNA, and ITS) as well as extant and fossil morphology were used to reconstruct the phylogeny and biogeographic history of an intercontinentally disjunct plant group, the cornelian cherries of Cornus (dogwoods). The study tests previous hypotheses on the relative roles of two Tertiary land bridges, the North Atlantic land bridge (NALB) and the Bering land bridge (BLB), in plant migration across continents. Three approaches, the Bayesian, nonparametric rate smoothing (NPRS), and penalized likelihood (PL) methods, were employed to estimate the times of geographic isolations of species. Dispersal and vicariance analysis (DIVA) was performed to infer the sequence and directionality of biogeographic pathways. Results of phylogenetic analyses suggest that among the six living species, C. sessilis from western North America represents the oldest lineage, followed by C. volkensii from Africa. The four Eurasian species form a clade consisting of two sister pairs, C. mas– C. officinalis and C. chinensis–C. eydeana. Results of DIVA and data from fossils and molecular dating indicate that the cornelian cherry subgroup arose in Europe as early as the Paleocene. Fossils confirm that the group was present in North America by the late Paleocene, consistent with the DIVA predictions that, by the end of the Eocene, it had diversified into several species and expanded its distribution to North America via the NALB and to Africa via the last direct connection between Eurasia and Africa prior to the Miocene, or via long-distance dispersal. The cornelian cherries in eastern Asia appear to be derived from two independent dispersal events from Europe. These events are inferred to have occurred during the Oligocene and Miocene. This study supports the hypothesis that the NALB served as an important land bridge connecting the North American and European floras, as well as connecting American and African floras via Europe during the early Tertiary.

We used DNA sequences of lecithotrophic monodontine topshells, belonging to the genera Diloma, Melagraphia, and Austrocochlea, to ascertain how this group became established over a large area of the South Pacific Ocean. The phylogeny of the topshells was estimated using portions of two mitochondrial genes (16S and cytochrome oxidase 1) and one nuclear gene (actin). A range of divergence rates was used to estimate the approximate timing of cladogenetic events within their phylogenetic tree. These estimates allow us to unambiguously reject vicariant explanations for several major divergence events and to infer several dispersal events across wide stretches of ocean. The first were two initial dispersal events from Australia (1) to an area between Samoa and Japan and (2) to New Zealand. Subsequently, at least one, and possibly two, recent eastward dispersals took place from New Zealand to Chile and the Juan Fernandez Islands, and one further dispersal occurred from somewhere in the tropical Pacific to Samoa. Moreover, owing to the short-lived nature of the topshell larvae, transoceanic larval dispersal is unlikely. The apparent paradox of a short larval phase and broad geographic range suggests that dispersal most probably occurred by rafting of adults on a suitable platform such as macroalgae; indeed, naturally buoyant bull kelp is the natural habitat of the most geographically widespread species in this group. Our molecular phylogenies imply that, despite of being an unlikely event, adult rafting in ocean currents has occurred on several occasions throughout the evolutionary history of topshells, resulting in their wide present-day distribution.

On the small oceanic island of Chichijima, two endemic species of land snails, Mandarina mandarina and M. chichijimana, have discrete distributions separated by a hybrid zone. This study investigates the potential of hybridization as a source of morphological novelty in these snails. Mandarina mandarina possesses a shell with a higher whorl expansion rate and a smaller protoconch than M. chichijimana, relative to shell size. The number of whorls and shell size of M. mandarina do not differ from those of M. chichijimana, because the effect of higher expansion rate on number of whorls and size of the former is compensated for by its smaller protoconch. The whorl expansion rate and protoconch diameter of the individuals from the hybrid populations are intermediate or typical of either of the two species, and their average values show clinal changes along the hybrid zone. However, the hybrid populations include exceptionally high shells with many whorls and flat shells with few whorls, which are never found in the pure populations of either species. In addition, gradual increases in variance in shell height and number of whorls were found from the edges to the center of the hybrid zone. A combination of low expansion rate (typical of M. chichijimana) and a small protoconch (typical of M. mandarina) produces a shell with an extremely large number of whorls because of the geometry of shell coiling. However, the combination of high expansion rate and a large protoconch produces a shell with an extremely small number of whorls. Because of the correlation between the number of whorls and shell height, shells with an exceptional number of whorls possess an extraordinarily high or flat spire. Hybrids can inherit a mosaic of characters that, as they play out during growth, lead to novel adult morphologies. These findings emphasize the importance of hybridization as a source of morphological variation and evolutionary novelty in land snails.

In Drosophila melanogaster, exposure of females to low temperature and shortened photoperiod can induce the expression of reproductive quiescence or diapause. Diapause expression is highly variable within and among natural populations and has significant effects on life-history profiles, including patterns of longevity, fecundity, and stress resistance. We hypothesized that if diapause expression is associated with overwintering mechanisms and adaptation to temperate environments, the frequency of diapause incidence would exhibit a latitudinal cline among natural populations. Because stress resistance and reproductive traits are also clinal in this species, we also examined how patterns of fecundity and longevity varied with geography and how stress resistance and associated traits differed constitutively between diapause and nondiapause lines. Diapause incidence was shown to vary predictably with latitude, ranging from 35% to 90% among natural populations in the eastern United States Survivorship under starvation stress differed between diapause and nondiapause lines; diapause phenotypes were also distinct for total body triglyceride content and the developmental distribution of oocytes in the ovary following stress exposure. Patterns of longevity, fecundity, and ovariole number also varied with geography. The data suggest that, for North American populations, diapause expression is functionally associated with overwintering mechanisms and may be an integral life-history component in natural populations.

We generated mitochondrial DNA (mtDNA) sequence data from 402 individuals of the fire ant Solenopsis invicta collected from 11 native populations and analyzed these data using a combination of demographic, phylogenetic, and phylogeographic methods to infer features of the evolutionary history of this species. Prior expectations regarding high levels of genetic structure and isolation by distance among populations were supported by the data, but we also discovered several unanticipated patterns. Our analyses revealed a major genetic break between S. invicta mtDNA haplotypes that coincides with the Mesopotamia wetlands region of South America, resulting in two higher level nested clade groupings. In addition, we identified contrasting patterns of genetic differentiation within these two major groups, which may reflect differences in connectivity of suitable habitat in different parts of the native range of S. invicta. Our study represents the first attempt to understand the phylogeographic history of S. invicta across its native range.

Large-bodied species of hosts often harbor large-bodied parasites, a pattern known as Harrison's rule. Harrison's rule has been documented for a variety of animal parasites and herbivorous insects, yet the adaptive basis of the body-size correlation is poorly understood. We used phylogenetically independent methods to test for Harrison's rule across a large assemblage of bird lice (Insecta: Phthiraptera). The analysis revealed a significant relationship between louse and host size, despite considerable variation among taxa. We explored factors underlying this variation by testing Harrison's rule within two groups of feather-specialist lice that share hosts (pigeons and doves). The two groups, wing lice (Columbicola spp.) and body lice (Physconelloidinae spp.), have similar life histories, despite spending much of their time on different feather tracts. Wing lice showed strong support for Harrison's rule, whereas body lice showed no significant correlation with host size. Wing louse size was correlated with wing feather size, which was in turn correlated with overall host size. In contrast, body louse size showed no correlation with body feather size, which also was not correlated with overall host size. The reason why body lice did not fit Harrison's rule may be related to the fact that different species of body lice use different microhabitats within body feathers. More detailed measurements of body feathers may be needed to explore the precise relationship of body louse size to relevant components of feather size. Whatever the reason, Harrison's rule does not hold in body lice, possibly because selection on body size is mediated by community-level interactions between body lice.

Hybrid viability decreases with divergence time, a pattern consistent with a so-called speciation clock. However, the actual rate at which this clock ticks is poorly known. Most speciation-clock studies have used genetic divergence as a proxy for time, adopting a molecular clock and often far-distant calibration points to convert genetic distances into age. Because molecular clock assumptions are violated for most genetic datasets and distant calibrations are of questionable utility, the actual rate at which reproductive isolation evolves may be substantially different than current estimates suggest. We provide a robust measure of the tempo at which hybrid viability declines with divergence time in a clade of freshwater fishes (Centrarchidae). This incompatibility clock is distinct from a speciation clock because speciation events in centrarchids appear to be driven largely by prezygotic isolation. Our analyses used divergence times estimated with penalized likelihood applied to a phylogeny derived from seven gene regions and calibrated with six centrarchid fossils. We found that hybrid embryo viability declined at mean rate of 3.13% per million years, slower than in most other taxa investigated to date. Despite measurement error in both molecular estimated ages and hatching success of hybrid crosses, divergence time explained between 73% and 90% of the variation in hybrid viability among nodes. This high correlation is consistent with the gradual accumulation of many genetic incompatibilities of small effect. Hybrid viability declined with the square of time, consistent with an increasing rate of accumulation of incompatibilities between divergent genomes (the snowball effect). However, the quadratic slope is due to a lag phase resulting from heterosis among young species pairs, a phenomenon rarely considered in predictions of hybrid fitness. Finally, we found that reciprocal crosses often show asymmetrical hybrid viabilities. We discuss several alternative explanations for this result including possible deleterious cytonuclear interactions. Speciation-clock studies have been a small cottage industry recently, but there are still novel insights to be gained from analyses of more taxonomic groups. However, between-group comparisons require more careful molecular-clock calibration than has been the norm.

Molecular clock methods allow biologists to estimate divergence times, which in turn play an important role in comparative studies of many evolutionary processes. It is well known that molecular age estimates can be biased by heterogeneity in rates of molecular evolution, but less attention has been paid to the issue of potentially erroneous fossil calibrations. In this study we estimate the timing of diversification in Centrarchidae, an endemic major lineage of the diverse North American freshwater fish fauna, through a new approach to fossil calibration and molecular evolutionary model selection. Given a completely resolved multi-gene molecular phylogeny and a set of multiple fossil-inferred age estimates, we tested for potentially erroneous fossil calibrations using a recently developed fossil cross-validation. We also used fossil information to guide the selection of the optimal molecular evolutionary model with a new fossil jackknife method in a fossil-based model cross-validation. The centrarchid phylogeny resulted from a mixed-model Bayesian strategy that included 14 separate data partitions sampled from three mtDNA and four nuclear genes. Ten of the 31 interspecific nodes in the centrarchid phylogeny were assigned a minimal age estimate from the centrarchid fossil record. Our analyses identified four fossil dates that were inconsistent with the other fossils, and we removed them from the molecular dating analysis. Using fossil-based model cross-validation to determine the optimal smoothing value in penalized likelihood analysis, and six mutually consistent fossil calibrations, the age of the most recent common ancestor of Centrarchidae was 33.59 million years ago (mya). Penalized likelihood analyses of individual data partitions all converged on a very similar age estimate for this node, indicating that rate heterogeneity among data partitions is not confounding our analyses. These results place the origin of the centrarchid radiation at a time of major faunal turnover as the fossil record indicates that the most diverse lineages of the North American freshwater fish fauna originated at the Eocene-Oligocene boundary, approximately 34 mya. This time coincided with major global climate change from warm to cool temperatures and a signature of elevated lineage extinction and origination in the fossil record across the tree of life. Our analyses demonstrate the utility of fossil cross-validation to critically assess individual fossil calibration points, providing the ability to discriminate between consistent and inconsistent fossil age estimates that are used for calibrating molecular phylogenies.

Evolutionary lineages differ with regard to the variety of forms they exhibit. We investigated whether comparisons of morphological diversity can be used to identify differences in ecological diversity in two sister clades of centrarchid fishes. Species in the Lepomis clade (sunfishes) feed on a wider range of prey items than species in the Micropterus clade (black basses). We quantified disparity in morphology of the feeding apparatus as within-clade variance on principal components and found that Lepomis exhibits 4.4 and 7.4 times more variance than Micropterus on the first two principal components. However, lineages are expected to diversify morphologically and ecologically given enough time, and this pattern could have arisen due to differences in the amount of time each clade has had to accumulate variance. Despite being sister groups, the age of the most recent common ancestor of Lepomis is approximately 14.6 million years ago and its lineages have a total length of 86.4 million years while the age of the most recent common ancestor of Micropterus is only about 8.4 million years ago, and it has a total branch length of 42.9 million years. We used the Brownian motion model of character evolution to test the hypothesis that time of independent evolution of each clade's lineages accounts for differences in morphological disparity and determined that the rates of evolution of the first two principal components are 4.4 and 7.7 times greater in Lepomis. Thus, time and phylogeny do not account for the differences in morphological disparity observed in Lepomis and Micropterus, and other diversity-promoting mechanisms should be investigated.

Animal color pattern phenotypes evolve rapidly. What influences their evolution? Because color patterns are used in communication, selection for signal efficacy, relative to the intended receiver's visual system, may explain and predict the direction of evolution. We investigated this in bowerbirds, whose color patterns consist of plumage, bower structure, and ornaments and whose visual displays are presented under predictable visual conditions. We used data on avian vision, environmental conditions, color pattern properties, and an estimate of the bowerbird phylogeny to test hypotheses about evolutionary effects of visual processing. Different components of the color pattern evolve differently. Plumage sexual dimorphism increased and then decreased, while overall (plumage plus bower) visual contrast increased. The use of bowers allows relative crypsis of the bird but increased efficacy of the signal as a whole. Ornaments do not elaborate existing plumage features but instead are innovations (new color schemes) that increase signal efficacy. Isolation between species could be facilitated by plumage but not ornaments, because we observed character displacement only in plumage. Bowerbird color pattern evolution is at least partially predictable from the function of the visual system and from knowledge of different functions of different components of the color patterns. This provides clues to how more constrained visual signaling systems may evolve.

Although the function of ornamental traits in males has been the focus of intensive research for decades, expression of such traits in females has received much less study. Eastern bluebirds (Sialia sialis) display structurally based ultraviolet/blue and melanin-based chestnut plumage, and in males this plumage coloration is related to both reproductive success and competitive ability. Compared to males, female bluebirds show a subdued expression of blue and chestnut ornamental coloration, and we used a combination of an aviary nutritional-stress experiment and four years of field data to test the hypothesis that coloration functions as a signal of female quality. First, we tested the effect of food intake on expression of structural and melanin coloration in female eastern bluebirds to determine whether structural or melanin coloration are condition-dependent traits. Females that were given ad libitum access to food displayed more ornamented structural coloration than females on a food-restricted diet, but there was no effect of the experiment on melanin ornamentation. Second, we used field data to assess whether female ornamentation correlated with measures of mate quality and parental effort. The structural coloration of females predicted first egg date, maternal provisioning rates, and measures of reproductive success. These data indicate that structural coloration is dependent on nutritional condition and suggest that sexual selection is acting on structurally based plumage coloration in female eastern bluebirds.

We studied the potential for response to selection in typical physiological-thermoregulatory traits of mammals such as maximum metabolic rate (MMR), nonshivering thermogenesis (NST) and basal metabolic rate (BMR) on cold-acclimated animals. We used an animal model approach to estimate both narrow-sense heritabilities (h²) and genetic correlations between physiological and growth-related traits. Univariate analyses showed that MMR presented high, significant heritability (h² = 0.69 ± 0.35, asymptotic standard error), suggesting the potential for microevolution in this variable. However, NST and BMR presented low, nonsignificant h², and NST showed large maternal/common environmental/nonadditive effects (c² = 0.34 ± 0.17). Heritabilities were large and significant (h² > 0.5) for all growth-related traits (birth mass, growth rate, weaning mass). The only significant genetic correlations we found between a physiological trait and a growth-related trait was between NST and birth mass (r = −0.74; P < 0.05). Overall, these results suggest that additive genetic variance is present in several bioenergetic traits, and that genetic correlations could be present between those different kinds of traits.

Interspecific aggression originating from mistaken species recognition may cause selection on secondary sexual characters, but this hypothesis has remained untested. Here we report a field experiment designed to test directly whether interspecific aggression causes selection on secondary sexual characters, wing spots, in wild damselfly populations. Males of Calopteryx virgo are more aggressive toward males of C. splendens with large than with small wing spots. This differential interspecific aggression may cause negative selection on wing spot size. Indeed, our results show that directional survival selection on wing spot size of C. splendens males was changed by experimental removal of C. virgo males. Without removal, directional selection went from positive to negative with increasing relative abundance of C. virgo males. In populations where C. virgo males were removed, this relationship disappeared. These results verify that interspecific aggression can cause negative selection on sexual characters. Thus, interspecific aggression has the potential to cause divergence on these characters between two species offering an alternative explanation for reinforcement for generating character displacement in secondary sexual characters.

The absence of continued evolutionary change despite the presence of genetic variation and directional selection is very common. Genetic correlations between traits can reduce the evolvability of traits. One intriguing example might be found in a sexual conflict over sexually dimorphic traits: a common genetic architecture constrains the response to selection on a trait subjected to sexually asymmetric selection pressures. Here we show that males and females of the mealworm beetle Tenebrio molitor differ in the quantitative genetic architecture of four traits related to immune defense and condition. Moreover, high genetic correlations between the sexes constitute a genetic constraint to the evolution of sexual dimorphism in immune defense. Our results suggest a general mechanism by which sexual conflict can promote evolutionary stasis. We furthermore show negative genetic correlations, strong indications of trade-offs, between immune traits for two pairs of traits in females.

Comparative studies provide correlational evidence of morphological adaptations for high locomotor performance, such as the classical indicators of cursoriality in mammals, long limbs and high metatarsal/femur ratios. More recently, enlarged femoral condyles have been suggested as an adaptation for high endurance running in the genus Homo. Asymmetry of locomotor appendages should adversely affect locomotor abilities, but this has not been studied in a rigorous evolutionary context. We used experimental evolution to test for morphological adaptations associated with high voluntary wheel running in selectively bred lines of mice. Surprisingly, the classical indicators of cursoriality had not evolved in concert with high activity levels. Instead, high runners had larger femoral condyles and reduced directional asymmetry of hindlimb bones. We hypothesize that greater limb symmetry and larger femoral heads are general adaptations associated with sustained, high-speed locomotion.